A standard for representing ownership of non-fungible tokens (NFTs), a special type of cryptographic token on the Ethereum blockchain.
ERC721 is an Ethereum Improvement Proposal introduced in 2018. It's an inheritable Solidity smart contract standard, meaning that developers can import it from the OpenZeppelin library into their smart contracts to easily create NFTs. NFTs are tokens that we can use to represent ownership of unique items. They let us tokenize things like art, collectibles, even real estate.
CryptoKitties, a blockchain game on Ethereum, uses ERC721 tokens to represent each unique digital cat.